1. Field of the Invention
The present invention relates to an electronic apparatus, a data processing method, a data processing system, and a computer readable storage medium, and more particularly to techniques of protecting copyright of various digital information, a digital information distribution system using such copyright protecting techniques, and an electronic commerce system.
2. Related Background Art
With recent developments of computer networks and wide spread of inexpensive and high performance computers, it is expected that services (called electronic commerce) of selling and buying commodities on networks will be prevailing. Commodities sold and bought in electronic commerce are digital data such as still images.
However, there are some problems to be solved in order to practice electronic commerce. For example, digital data has a general property that a number of perfect copies of the digital data can be made easily, and that the contents thereof can be easily altered. There is therefore a possibility that a user who bought a commodity of digital data illegally makes copy data of the same quality and redistributes it to another user. In this case, a copyright holder of the commodity or an agent (hereinafter called a sales agent) entrusted by the copyright holder cannot receive a proper countervalue to be paid for the commodity.
If the copyright holder or sales agent (hereinafter both are collectively called a server which can legally distribute commodities of digital data) once sends a commodity to a buyer, the copyright of the commodity may be infringed because it is impossible to completely prevent illegal copies of the commodity and alteration of the contents of the commodity.
As techniques of solving such problems associated with electronic commerce, techniques called “digital watermark” has been studied.
Digital watermark techniques embed copyright information of digital data and user information of a buyer in the digital data itself by processing original digital data and making it invisible. By incorporating the digital watermark techniques, if an illegal copy is found, it is possible to identify the person who redistributed the illegal copy.
As an approach to realizing the digital watermark techniques now under developments, there are a method of processing a specific frequency band by using discrete cosine transform, Fourier transform, wavelet transform or the like, and a method of directly processing the luminance value of each pixel.
Security and reliability of digital watermark techniques rely upon invisible information embedded in digital data and upon an inability of destroying key information or altering the contents thereof unless the key information regarding an information embedded location and an embedding intensity is known.
For example, in B. Pfitmann and M. Waidner: “Asymmetric Fingerprinting, EUROCRYPT 96”, a system is proposed which can identify illegal distribution by embedding user information of a buyer of each image. However, even with this system, a user can illegally destroy key information or alter the contents thereof if the key information used by embedding the user information is made public.
One example of a system using digital watermark techniques is illustrated in FIG. 1.
Referring to FIG. 1, an digital watermark embedding circuit 111 on an embedding side 110 embeds information D in original image data G by using key information k. The key information k is necessary for extracting the embedded information D, and is, for example, an embedding location of the embedding information D, an embedding intensity, and the like. The image data G embedded with the embedding information D is transmitted to an external site as digital watermark image data.
An digital watermark extracting circuit 121 on an extracting side 120 extracts the embedded information D from the digital watermark image data by using key information k same as the key information on the embedding side 110.
As above, with the system shown in FIG. 1, the embedding side 110 and extracting side 120 embed and extract the digital watermark embedding information D, by using the key information k shared by both sides.
It is necessary for both the embedding side 110 and extracting side 120 to keep in secret the key information regarding an embedding location of the embedding information D, an embedding intensity and the like, in order to prevent the above-described illegal operations.
It is therefore presumed that the embedded information D can be extracted only by key information management facilities which generated the key information k or by special inspection facilities which can know the key information. The right of a copyright holder can be protected by supervising illegally redistributed digital data or digital data whose contents were altered.
From analogy to encryption techniques, such a system configuration can be considered as a scheme like the common key-cryptosystem which uses the same key for an enciphering key and a deciphering key.
With the system shown in FIG. 1, however, it is necessary to keep in secret the key information k necessary for extracting the embedded information D both at the embedding side 110 and extracting side 120, so that the key information cannot be transferred freely via a network. Furthermore, the embedded information D can be extracted only by the above-described special facilities.
Since general users cannot extract the embedded information D freely, each user cannot confirm the copyright contents and legitimacy of digital data acquired from an external site. This is very inconvenient for the user.
In order to solve the above problems, the key information k necessary for extracting the embedded information D is required to be made public to general users including a maker of the electronic apparatus on the extracting side 120.
However, the publicized key information k may cause infringement of the copyright of digital data through the above-described illegal operations. Namely, if the key information k is made public in a simple manner, any one of general users can confirm the embedded information D. Therefore, the copyright of digital data cannot be protected sufficiently because of the embedded information D allowed to be confirmed by anybody.
Conventional digital watermark techniques do not therefore propose an approach to satisfying both free extraction of the embedded information D of an digital watermark by each user and protection of the copyright of digital data, without keeping in secret the key information k necessary for extraction of the embedded information D. Furthermore, the conventional digital watermark techniques do not propose copyright protection techniques using such digital watermark techniques, electronic commerce systems and digital information distribution systems using such copyright protection techniques.